1 Understanding Route Redistribution ICNP 2007 October 17 th, 2007 Franck Le, Geoffrey G. Xie, Hui Zhang

2 Internetwork and Routing Common view: –Intra-domain routing using OSPF, RIP –Inter-domain routing using BGP In reality, internetworking is much more complex –ISP networks: OSPF routes to be redistributed into BGP (and vice versa) –Enterprise networks: When BGP is not used, needs mechanism to distribute routes among OSPF, RIP, EIGRP domains Also, needs to distribute routes among multiple OSPF domains

3 What is Route Re-Distribution (RR)? router ospf 27 redistribute rip metric 200 subnets route-map rip2ospf distance ospf external 200 ! route-map rip2ospf permit 100 match ip address 100 set tag 22 set metric-type-1 A B D E Office branch 1Office branch 2 RIPOSPF RIPOSPFLocal FIB C By default, OSPF routers have no visibility of RIP routers

4 How Does RR Compare to BGP? In many scenarios, RR, not BGP, is used to interconnect network domains, Even when BGP is used, RR is required to connect BGP and IGP RR can implement policy, like BGP Unlike BGP, RR is NOT a protocol –RR is just a configuration mechanism, used separately at each router RR is more commonly used than BGP, but much less understood, and much more error-prone

5 Problem Statements Given an internetwork with RR configurations, what are the loop-free and convergence properties? What are the guidelines of using RR if one wants to have loop-free and convergent internetwork?

6 Synthesis of the Paper Model that reasons about the loop-free and convergence properties Sufficient condition to guarantee loop-free and convergence properties

7 Outline 1.Introduction to Route Redistribution (RR) 2.Illustration of routing anomalies 3.A Model for RR 4.Sufficient condition for loop-free and convergent RR

8 Route Selection Process A B C D E Office branch 1Office branch 2 RIPOSPF RIP FIB OSPFLocal P P P Signaling Data path

9 Route Selection Process A B C D E Office branch 1Office branch 2 RIPOSPF RIP FIB OSPFLocal Selected routing process PP P P P Signaling Data path OSPF /1

10 FIB Route Redistribution Process A B C D E Office branch 1Office branch 2 RIPOSPF RIPOSPFLocal /1 OSPF RIP Update P P Signaling Data path

11 Outline 1.Introduction to Route Redistribution (RR) 2.Illustration of routing anomalies 3.A Model for RR 4.Sufficient condition for loop-free and convergent RR

12 Instabilities Wide range of possible routing instabilities No general guideline to configure RR

13 RIP OSPF RIP Formation of Routing Loops A B C D E RIP(120)OSPF(110) OSPFLocal FIB RIP OSPF Local FIB P Next-hop: B Next-hop: C Next-hop: E Next-hop: D P P P Signaling Data path

14 Outline 1.Introduction to Route Redistribution (RR) 2.Illustration of routing anomalies 3.A Model for RR 4.Sufficient condition for loop-free and convergent RR

15 Challenges Too many network elements –Hundreds or thousands of routers Different router processing order –Routers may process signaling messages in different order (message delay, router load) – Different order can result in different outcome

16 Solutions Too many network elements –Abstractions: routing instances –Logics: route selection, RR, network-wide RR Different router processing order –Activation sequence 1 1 L. Gao and J. Rexford, Stable Internet Routing Without Global Coordination, in Proc. ACM SIGMETRICS, 2000

17 A Model for RR Abstracts the dynamic exchange of routing information for a prefix P Allows to predict paths

18 Route Propagation Graph Routing instance Originating routing instance Configured redistribution Actual redistribution Route vs. no route Variables: CL, S 2 (110) 1 (120) 1 (120) 2 (110) 80, A, 90 1 (120) 2 (110) 80, A, 90 1 (120) 2 (110) 80, A, 90 2 (110)

19 Illustration of Model 2 OSPF1 (110) 3 RIP (120) 4 OSPF2 (110) F F L L H H E E 0 Local (0) 1 RIP (120) A A BCDE F GH I K L M N J RIP OSPF1OSPF2 P

20 Illustration of Model Sequence 1 Signaling Data path 1 RIP (120) 2 OSPF1 (110) 3 RIP (120) 4 OSPF2 (110) F F L L H H E E 0 Local (0) A 1 RIP (120) 2 OSPF1 (110) 3 RIP (120) CL(t=0) = {A}CL(t=1) = {E, F} CL(t=2) = {E, L}CL(t=3) = {E, H} CL(t=4) = {E}CL(t=5) = {A, F}CL(t=6) = { } S(t=1) = {A}S(t=2) = {F}S(t=3) = {L} S(t=4) = {H} S(t=5) = {E}S(t=6) = {A, F} 4 OSPF2 (110)

21 Route Redistribution Configuration - Cycle Detection (RRC-CD) Problem Given a RR configuration, determining whether there is an activation sequence such that the redistributions converge to state including a cycle of active redistributions is NP-hard

22 Outline 1.Introduction to Route Redistribution (RR) 2.Illustration of routing anomalies 3.A Model for RR 4.Sufficient condition for loop-free and convergent RR

23 Sufficient condition for safety Pruning of Route Propagation Graph –For each redistributing router, only conserve redistributions from the routing processes with lowest administrative distances Rationale –Focus on preferred redistributions 1 (100) 2 (70) 3 (120) 4 (90) AAA

24 Sufficient condition If resulting graph satisfies 1.Every redistributing router redistributes from a single routing instance (predictable outcome) 2.For all vertice, there is a redistribution path from a originating vertex(active redistribution) 3.The graph is acyclic(no cycle) Then, the redistributions converge to an acyclic routing state  No route oscillations  No forwarding loops

25 Application of Sufficient Condition 1 RIP (120) 2 OSPF1 (110) 3 RIP (120) 4 OSPF2 (110) F F L L H H E E 0 Local (0) A

26 Application of Sufficient Condition Modifications 1 RIP (120) 2 OSPF1 (110) 3 RIP (120) 4 OSPF2 (110) 80, F F, 80 L L H H 80, E E, 80 0 Local (0) A

27 Application of Sufficient Condition Pruning 1 RIP (120) 2 OSPF1 (110) 3 RIP (120) 4 OSPF2 (110) 80, F F, 80 L L H H 80, E E, 80 0 Local (0) A

28 Application of Sufficient Condition Pruning 1 RIP (120) 2 OSPF1 (110) 3 RIP (120) 4 OSPF2 (110) 80, FL H 0 Local (0) A 80, E

29 Application of Sufficient Condition 1.Every redistributing router is redistributing from a single routing instance. 2.For all vertice, there is a redistribution path from a originating vertex. 3.The graph is acyclic. 1 RIP (120) 2 OSPF1 (110) 3 RIP (120) 4 OSPF2 (110) 80, FL H 0 Local (0) A 80, E

30 Summary Internetwork is far more complex with RR than the conceptual model of BGP/OSPF RR serves a fundamental need, but is not well-understood or even well-designed First formal study route-free and convergence properties of RR internetwork –Model –Sufficient condition

31 Future Work If one were to re-design the RR, what should be the solution that supports all the RR applications but avoid the pitfalls?